If You Build It They Come — But What Happens If You Take It Away?

If you build it…it will fill up — a truism for both roads and bikeways.  But if it isn’t there, or even if it was once there and you take it away, the traffic seems to go away as well…which may be the most important fact about traffic planning that you will never hear from the highway lobby. 

I spent two hours during a recent afternoon trying to get out of Boston through the Big Dig tunnel.  Traffic wasn’t being bottled up by an accident; it just always seems congested in the late afternoon.  Funny thing is that I don’t remember traffic being so bad during those endless years when Big Dig construction was being so corruptly mismanaged and lanes were always being shut down.  Somehow, people found other ways (or times) to get where they needed to go.

The Big Dig is a local proof of the reality, confirmed by anecdotes and research from around the world, that traffic expands to fill the available space.  As the movie says, “if you build it, they will come”…and come…and come…until the congestion you were trying to address through additional construction is recreated and you are back where you started with air pollution, noise, long waits, and frustrated commuters.

But, the Big Dig is also proof of the opposite reality:  if it goes away so do they.  As traffic capacity diminishes, people find other ways to get around and the much feared reduced-capacity gridlock never materializes (or clears up after an adjustment period).  This process, which traffic researchers call “shrinkage,” explains how New York and other major cities around the world have been able to reduce their dependence on cars — usually containing only one person — by reducing the number or width of traffic lanes and parking spots without hindering mobility or business.

Mode change doesn’t require that huge numbers of people abandon their cars for public transportation or bicycles or even car pools.  Huge impacts are the accumulation of small actions, so all that it takes is a small change at the margins – one fewer single-occupancy car trip a week per household, one more use of the bike instead of a car to run to the store, one pleasant walk to a friend’s house.

Catalyzing this kind of change requires changing the context within which people make travel decisions.  Fortunately, just as with highways, we’ve learned that the available infrastructure shapes usage in other modes of transportation as well.  If you do a good job expanding public transit, more people will ride the subways and buses.  If you build more bike lanes and off-road paths, cycling will become mainstream.  If you improve sidewalks and intersections, people will be happy to walk.

It is for this reason that reshaping our infrastructure is the key to reshaping our transportation behaviors. Urging people to bike or walk instead of driving their cars doesn’t hurt.  But the foundation for a better future rests on the solid reality of facility development.

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